1. Field of the Invention
This invention relates generally to an improved method and package for dissipating heat generated by a semiconductor chip and, more particularly, to an improved method and package for dissipating a large amount of thermal energy from an integrated circuit chip.
2. Background of the Invention
In integrated circuit chips that generate a relatively large amount of heat (for example, because of the presence and use of a high density or amount of semiconductor components which may include high heat producing power devices that are formed in and become a part of the integrated chip structure), the heat must be conducted away from the chip. Otherwise, the rise in temperature can compromise or detrimentally affect the performance of the circuit and may even cause the destruction of one or many components thereby creating a circuit failure. Numerous techniques have been devised to improve the dissipation of heat away from the integrated circuit chip and thereby achieve, in effect, the cooling of the package containing the integrated circuit chip. Thermally coupling the integrated circuit chip to a heat sink is a frequently performed solution to the problem of heat dissipation. However, the package housing is frequently comprised of a poor thermal conductor such as a plastic or resin dielectric or insulating material used as a housing or encapsulating material, thereby limiting the effectiveness of a heat sink in contact with the dielectric portion of the housing package.
It is known in the related art to couple the integrated circuit chip to a thermal conductor or heat sink element that is part of the package housing to permit superior conduction of heat away from the integrated circuit package. However, the packages that have a heat sink element or, effectively, a cooling plate directly and mechanically coupled to the integrated circuit chip have, in the past, been difficult to fabricate and effectively use with integrated circuit chips. In addition, the electrically conducting leads of the package that are used for electrically coupling the various electronic components or circuit functions of the integrated circuit chip with the external circuitry typically require and use very fine line electrically conducting wires which are very fragile and need to be securely attached to the electrically conducting leads of the package. Problems have been encountered in providing leadframe conductors that can provide good electrical contact (via the fine line wires) to desired portions of the chip, that can also provide a good mechanical and thermal connection to a cooling plate, and that can further provide electrical isolation between a metal heat sink or cooling plate and the individual leadframe conductors.
A need was therefore felt for an improved integrated circuit package in which the integrated circuit chip is coupled to a heat dissipating element or heat sink member to thereby provide a cooling plate. The improved package is preferably fabricated using leadframe conductors or the leadframe technology and provides the solutions to the above identified problems. Moreover, the leadframe conductors are preferably coated, at least on the portions thereof in contact with the heat sink member, with an electrically insulating, thin, varnish material coating, such as a Teflon-based lacquer, which still permits both good thermal and mechanical contact with the heat sink member or cooling plate. This varnish material is suitable for retaining electrical insulation between the leadframe conductors and the heat sink member or cooling plate and is stable and will not break down at higher temperatures in order to insure proper semiconductor (device or integrated circuit) operation at operating temperatures (i.e., approximately 175.degree. C.). Therefore, this varnish material is long lasting, durable and will not degrade or become damaged, especially when subjected to heating operations or procedures during the semiconductor package assembling process. Both mechanical and thermal stresses are appropriately tolerated or absorbed by this varnish material. More particularly, this varnish insulating material is suitable for being applied in a varnish-like fashion which is a significantly reliable, economical and rapid coating process. Further, this material can be applied or deposited at high accuracies (e.g., within .+-.0.10mm), especially when applied to metal leadframes. Still further, the metal leadframes with this varnish material coating can be mechanically sheared without detaching or striping off this varnish coating from the substrate or leadframe to which the coating is attached. Additionally, the metal lead frame with its attached thin electrically insulating, varnish material coating can still be dipped into baths for electrolythic plating operations, as desired, without damaging the varnish material coating.